HPAC - Modern Hydronics Fall 2026

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CONTENTS

Modern Hydronics

MH4 SYSTEM DESIGN

MH14

Flip Flop

NEW PRODUCTS

Tips on setting up a ‘load toggling’ design to satisfy heat calls for both an air handler and a radiant slab floor at the same time.

By John Siegenthaler

MH9 CASE STUDY

Cooling Under Control Reviewing a modern house project outside of Vernon, B.C. using a controls-first strategy to incorporate hydronic heating and cooling.

By Doug Picklyk

From the latest in air-to-water heat pumps to safety valves, electric boiler/storage tanks and hydronics design software.

MH16

It’s About Time As the industry continues down the road to electrification, there are ways to maximize hydronics system designs to minimize electrical costs.

By John Siegenthaler

MODERN HYDRONICS COVER PHOTO:VISUAL GENERATION/GETTY IMAGES

a supplement of Heating Plumbing Air Conditioning Magazine 111 Gordon Baker Road, Suite 400, Toronto, ON M2H 3R1 TEL: 416.442.5600 FAX: 416.510.5140 www.hpacmag.com

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MODERN HYDRONICS

FALL 2026

MH3

SYSTEM DESIGN

Modern Hydronics

FLIP FLOP Setting up a simple and cost-effective ‘load toggling’ design to satisfy heat calls for both an air handler and slab floor at the same time.

BY JOHN SIEGENTHALER

S

ome hydronic system designers choose to combine a heated slab floor on the ground level of a home with an air handler on the second story. Figure 1 (below) shows an example of such a system, which is supplied by a monobloc air-to-water heat pump.

For the sake of discussion, assume that air-to-water heat pump has a nominal 4-ton (48,000 Btu/h) heat output rating. At design load the air handler is rated for 20,000 Btu/h output when supplied with water at 120F, and the slab is designed to release 20,000 Btu/h when supplied with water at 105F.

To produce these different supply water temperatures the installer decides to set up the heat pump controls for a supply temperature of 120F, and he uses a motorized three-way mixing valve to reduce the water temperature to the floor heating circuits.

When only the air handler is operating the heat pump supplies the 120F water.

Whenever the heated slab is operating by itself the heat pump has no problem providing the required water temperature.

However, when the floor heating zone and the air handler are operating at the same time, and the outdoor air tempera-

ture is around -4C/24F, the fluid temperature leaving this heat pump can’t climb above 103F, regardless of how long it operates.

WHAT’S WRONG? All hydronic systems seek to operate at “thermal equilibrium.” That’s a steady state condition where the rate of heat production by the heat source is the same as the rate of heat dissipation by the load.

The greater the heat emitter surface area available to dissipate heat, the lower the water temperature will be when Continued on MH6

air handler

PRV

monobloc air-to-water heat pump

check valve

fluid feeder

mixing valve

Figure 1. A simple air-to-water system heating a slab floor and a hydronic air handler.

MH4

FALL 2026

slab floor heating

MODERN HYDRONICS

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ILLUSTRATION: JOHN SIEGENTHALER

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SYSTEM DESIGN

TIME SHARING A more practical approach, especially when correcting an already installed system, is to “share” the heat pump’s heat output between the two loads. I refer to this as “load toggling.”

It requires detection of when both loads are calling for operation at the same time. When this occurs, the controls need to allow only one load to operate for a specific time and then switch to the other load (and run only that load for another specific amount of time).

For example, when both loads in Figure 1 are calling for operation at the same time, the slab heating might be allowed to operate for 25 minutes with the air handler off, followed by the air handler running for 35 minutes with the slab off.

Load toggling can be accomplished using a repeat cycle time delay relay, such as the one shown in Figure 3.

This relay has multiple operating modes and settings for those modes. For the toggling application, the mode setting would be “flashing.” For the relay shown in Figure 3, the time over which each cycle of the relay’s contacts occurs is adjustable from 0.05 seconds to over 100 hours.

For this application, a time interval of several minutes is appropriate. For example, allowing the air handler to operate for 30 minutes with the no heat input to the slab, and then switch it off and allow slab heating to operate for the next 20 minutes with no heat input to the air handler. This 30/20 time interval MH6

FALL 2026

40000

slab heat output 20,000 Btu/hr at Ts = 105 ºF

105 ºF

20000

combined heat output air handler + slab heat pump output (Btu/hr)

balance point

30000

air handler output 20,000 Btu/hr at Ts = 120 ºF

10000 0

0 ºF

14 ºF

28 ºF

24 ºF

room air temperature = 70 ºF

50000

WHAT TO DO? One way to overcome this issue is to install a heat pump with a higher heating capacity. This would shift the balance point up and to the right, allowing for a higher supply water temperature to the air handler when both loads are operating at the same time.

However, that higher supply temperature still might not make it up to 120F under design load conditions.

Beyond this uncertainty is the daunting and expensive change out of the existing heat pump for a higher capacity unit. Doing so could also lead to short cycling under partial load conditions, especially if the larger heat pump is a single speed unit.

42 ºF

60000

56 ºF

70 ºF

outdoor air temperature (ºF)

Heat output (Btu/hr)

thermal equilibrium is achieved.

The graph in Figure 2 shows how the heat output of the floor slab and air handler will vary based on supply water temperature. The room air temperature is assumed to be 70F. At a supply water temperature of 103F the heating capacity of the heat pump is the same as the total heat dissipation ability of the distribution system.

That’s where this system achieves thermal equilibrium.

The floor heating zone is fine at this condition, but the output of the air handler is significantly lower than what was planned for based on the assumption that 120F supply water would be available.

70

80

90

100

110

Supply water temperature (ºF)

120

Figure 2. A graph illustrating how heat output of the floor slab and air handler will vary based on supply water temperature.

then repeats as long as both loads are calling for heat.

These time intervals are easily adjusted, and the relay in Figure 3 has double pole/double throw (DPDT) contacts, and it mounts into a standard 8-pin octal relay socket. It’s enabled to operate whenever 24 VAC is applied to the coil.

In Figure 4 you see a wiring schematic where the repeat cyFigure 3. Example of a repeat cycle time delay cle time delay relay (RCR) is comrelay, the Macromatic TR bined with four other single pole/ 6512U. double throw (SPDT) relays.

Relays (R1) and (R2) have 120 VAC coils, and are wired in parallel with their associated zone circulator. Their function is to “detect” when their associated zone circulator is called to operate by the multi-zone relay panel.

A normally open contact in relay (R1), designated as (R1-1), is wired in series with a normally open contact in relay (R2), designated as (R2-1). When both of these contacts are closed, 24VAC from the zone relay panel, or a separate 24VAC transformer, passes to the coil of the repeat cycle time delay relay (RCR), and it begins its first timing cycle.

The (RCR) relay has been configured so that the first timing cycle immediately sets the following contact status: • RCR-1 closes • R3 coil is energized • R3-1 N.C. opens (turning off the air handler circulator) • RCR-1 N.C. opens • R4 coil is off • R4-1 N.C. remains closed (allowing the slab circulator to run)

MODERN HYDRONICS

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Modern Hydronics

120 VAC multi-zone relay center zone 2

R2

R1 120 VAC coil

R3-1 N.C.

normally closed

pole #

relay name

120 VAC coil

R4-1 N.C.

RCR-2 N.C.

relay coil normally open relay contact normally closed relay contact

L N

slab air handler heating zone zone

24 VAC motorized mixing valve

24VAC

R1-1

zone 1

X

24 VAC coil R3

R2-1

X

24VAC

heat source heat enable

• R3-1 N.C. closes (allowing the air handler circulator to run) • RCR-1 N.C. closes • R4 coil is energized • R4-1 N.C. opens (turning off the slab circulator) At the end of the second cycle the above “toggling” actions repeat if both loads are still calling for heat.

When only one load is operating the repeat cycle time delay relay (RCR) is not energized, and relay contacts R3-1 N.C. and R4-1 N.C. are both closed. The active load is controlled solely by its associated thermostat.

RCR-1 RCR

RCR-1 N.C.

R4 24 VAC coil

Figure 4. A wiring schematic with a repeat cycle time delay relay (RCR) combined with four single pole/double throw (SPDT) relays.

At the end of the first timing cycle the status of the relay contacts change to the following: • RCR-1 opens • R3 coil is off

EASY & INEXPENSIVE Relays (R1), (R2), (R3), and (R4) are simple single pole double throw (SPDT) relays. The (R1) and (R2) relays must have 120 VAC rated coils, while (R3) and (R4) must have 24 VAC rated coils with 24 VAC coils.

The contacts in relays (R1) and (R2) are only handling a fraction of one amp in low voltage circuits. The contacts in relays (R3) and (R4) handle the current needed to operate one of the circulators. Most residential and light commercial zone pumps only require 1 to 2 amps.

You can see in Figure 5 a product that can be used for relays

HYDRONICS ON TAP

Continued on MH8

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MODERN HYDRONICS

FALL 2026 MH7 2026-07-24 9:17 AM

SYSTEM DESIGN thermostats

120 VAC

X

relay name

OMRON G2S-1-S relay coil

N.C.

com.

N.O.

Figure 6. The three connections for the contacts are grouped together.

(R3) and (R4). It’s an OMRON G2R-1-S, with 24VAC coil and contacts rated up to 10 amps. A very similar relay, OMRON G2R-1-T, has a 120 VAC coil, and can be used for relays (R1) and (R2).

You can buy these SPDT relays online in the price range of $10 to $15 each depending on quantity. They can be mounted on relay sockets or directly wired using quick connect terminals as shown in Figure 4.

These relays have clear plastic cases through which you can see the contacts.

The two contacts that are touching each other are the normally closed contacts.

The two contacts with a gap between them are the normally open contacts.

The two connections for the coil are grouped together. The three connections for the contacts are also grouped together, as shown in Figure 6.

These relays can be mounted in any orientation. Be sure to use an indelible marker to label each relay.

MH8

FALL 2026

24VAC

R2 R1-1

R1

R3-2 N.C.

normally open relay contact normally closed relay contact

R3

RCR-2 N.C.

relay coil

Figure 5. Example of a relay, an OMRON G2R-1-S with a 24VAC coil and contacts rated up to 10 amps.

multi-zone relay center

RCR-1

pole #

L N

24VAC

normally closed

R3-1

RCR-2 N.C.

zone 2

zone 1

X

R2-1

boiler heat enable

RCR

air handler zone

slab heating zone

24 VAC motorized mixing valve

Figure 7. Wiring design when using three relays.

Also be sure to leave a wiring diagram, such as the one in Figure 3, with the system documentation so that a future technician can see how the coils and contacts need to be arranged.

It’s worth mentioning that the same operating logic could be done using three relays rather than the 4 (SPDT) relays shown in Figure 3. However, one of those relays must be at least a double pole/ double throw (DPDT) relay. They’re available but are considerably more expensive than the (SPDT) relays mentioned earlier. Figure 7 shows the required wiring using three relays.

FINAL THOUGHTS If you apply this technique be sure the zones in the multi-zone relay centre are not configured for priority operation.

Relays other than those mentioned above, but with equivalent functionality, could be used. Be sure that the coil voltages for the relays are correct and ensure the current rating of the relays is suitable to handle the current required by the circulators.

I suggest wiring the relays as shown in MODERN HYDRONICS

the schematics and testing the operation of the overall control system prior to installing it on-site.

Load toggling is well applied in systems where one of the loads is a high thermal mass heat emitter such as a heated floor slab. The thermal mass of a heated slab tends to dampen out “pulsed” heat inputs with virtually no noticeable effects on comfort. Pulsed heat input to low thermal mass heat emitters such as the air handler will be more noticeable.

Still, as long as those low mass emitters can deliver the heating energy needed by the space averaged over a period of perhaps an hour, the average interior temperature should not vary more than it would with any low mass emitter operated by a thermostat. <> John Siegenthaler, P.E., has over 40 years of experience designing modern hydronic heating systems and is the author of Modern Hydronic Heating (4th edition) and Heating with Renewable Energy (visit hydronicpros.com).

WWW.HPACMAG.COM

CASE STUDY

Modern Hydronics

This custom 4,000 sq. ft. modern house uses indoor radiant heating and cooling.

COOLING UNDER CONTROL A custom home built near Vernon, B.C. performs as a showcase for air-to-water heating and cooling in a Canadian climate.

BY DOUG PICKLYK

ALL IMAGES COURTESY TEKMAR

F

ollowing over 20 years of working as a contractor in Canada, selling, designing, installing and servicing geothermal systems coupled with hydronics in boutique homes in the Nelson, B.C. area, around four years ago Scott Grimshaw (a native New Zealander) launched a product distribution and design consulting company, Granite Geohyrdronic Supply, with the goal of educating the market on how to design modern mechanical systems using heat pumps for hydronic heating and cooling.

Grimshaw established his new distribution business in Coldstream, B.C., a municipality directly southeast of Vernon and located at the northern end of Kalamalka Lake in the Okanagan Valley.

The relocation also initiated the conWWW.HPACMAG.COM

struction of his family home, a custombuilt 4,000 sq. ft. two-level house on a hill overlooking the lake.

A modern design in many ways, the building envelope approached passive house levels with an air leakage rate of close to 0.6 air changes per hour at 50 pascals (0.6 ACH50), and for the mechanical system he installed an air-towater heat pump feeding in-floor radiant on both levels along with fan coils to supplement heating and cooling and also provide the necessary humidity control.

“When we were building this house, I decided to put a system in that could be a showcase for young contractors and builders,” says Grimshaw. With plenty of glazing and full southwest exposure on both levels, he notes that it made a great MODERN HYDRONICS

test case for this kind of heating and cooling system.

SHOWCASE PROJECT His project also served as a way to display the products he was distributing, and it also turned out to be a perfect test site for a prototype of the Smart Heat Pump Control 291 being developed by Tekmar (headquartered in Vernon), a product line Granite also represents.

As an Enertech distributor, Grimshaw selected that company’s Advantage airto-water system for the project. The heat pump’s outdoor monobloc unit connects to an indoor hydro box with water lines, and then out to the zones.

With an inverter driven compressor in the outdoor unit and a variable speed Continued on MH10 FALL 2026

MH9

CASE STUDY

The home uses a monobloc air-to-water heat pump to supply the hot and cold water for the hydronic heating and cooling.

Smart Mix and Smart Heat Pump controls are mounted on the side wall. The floor standing boiler is for the swimming pool.

These well-insulated branches (right) lead to the fan coils, and in cooling mode deliver the coolest water available to help condense and remove humidity from the air.

circulator and a pressure-independent bypass on the indoor control, the system can self-adjust its heating and cooling output to the load requirements, and because of this it doesn’t require a

buffer tank to avoid short cycling potential. A space saving benefit for the mechanical room.

For the in-floor radiant, there are eight zones (four on each level), and the design

MH10

FALL 2026

MODERN HYDRONICS

called for three inches of insulation under the lower level four-inch slab. On the upper floor the tubing was encased in a twoinch pour. The PEX is layed out with six-inch spacing on centre. Both levels were covered with engineered hardwood.

“I’ve traditionally used six-inch spacing everywhere, and it works well with cooling because you get that even output,” says Grimshaw.

To complement the radiant systems, he installed Jaga fan coils (another brand he represents) to provide second stage heating and cooling and manage the humidity when cooling.

In some areas two zones share one fan coil (two downstairs bedrooms) and in the downstairs open space there’s a wall mounted panel radiator with a blower fan and condensate drain that takes care of that space.

The primary bedroom suite has a fan coil mounted in a bulkhead in the closet, along with a duct into the bathroom.

Grimshaw notes that you don’t use radiant floor cooling in bathrooms because WWW.HPACMAG.COM

Modern Hydronics there’s too much humidity created. In those zones the floor cooling is turned off at the controls.

The kitchen space on the upper floor has two ducts feeding an architecturally attractive long slot grill that satisfies the main floor, kitchen, dining and living area. “I always wanted a slot grill,” says Grimshaw.

The house has an ERV system, but it’s separate from the heating/cooling mechanics. The mechanical room also includes a Viessmann high mass boiler used primarily for the swimming pool. It was Grimshaw’s first pool, and he wanted to be able keep the temperature almost hot tub hot like, and he wanted to be able to keep swimming in the winter months.

COOLING CONTROL The system incorporates Tekmar’s new smart heat pump control that is also connected to Tekmar thermostats, zone valve controls and a smart mixing con-

trol. The controls allow remote monitoring via a mobile app. While hydronic heating controls have become straight forward for experienced hydronics techs, it’s the move to cooling that requires additional considerations.

For first stage cooling in the house, the zone thermostats initiate the radiant in floor system, but if the temperature inside increases and the radiant cooling can’t keep up, stage two brings on the fan coils. “We can have preset fan speeds on how much we want those fan coil motors to blow,” says Grimshaw.

As mentioned, the fan coils also serve to remove humidity from the air to prevent condensation inside the home. “The thermostats basically measure and control room temperature and relative humidity,” explains Mercedeh Fallahkhair, senior product manager with Tekmar.

“Using these values, the thermostats calculate the rooms’ dew point tempera-

ture, which is where condensation will form on cold surfaces.”

The dew point is communicated to the Tekmar smart heat pump control which measures both the outdoor air temperature as well as the heat pump’s supply and return water temperatures. This is where the smart mixing control (Smart Mix 295) comes into play.

For the radiant in floor, the mixing control regulates a three-way mixing valve keeping the floor water temperature approximately two degrees Fahrenheit (or one degree Celsius) above the highest dew point reported by the thermostats.

“This ensures condensation does not form on the piping, manifolds, or floor surface while still providing effective cooling,” notes Fallahkhair, who adds that the mixing valve is used to both regulate the water temperature to be above the dew point for cooling and for outdoor Continued on MH12

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MODERN HYDRONICS

FALL 2026 MH11 2026-07-30 3:12 PM

CASE STUDY reset during the heating season.

“I was really happy about how accurate that smart mix control was able to target temperature for the radiant” adds Grimshaw. Only one mixing valve was required to take care of all the radiant zones on both levels of the home.

For the fan coils, there is no mixing required. “We want to run really cold water across those radiators and pull the humidity from the home and drop it into the condensate pan and run that down the drain,” says Grimshaw.

There are Belimo two-way valves for the hydronic fan coils. When a thermostat makes that second stage call it closes the contacts and opens the flow to the fan coil, and that is set for whatever flow rate the fan coil requires.

Because of the colder water used for this application all the pipes have been well insulated to prevent condensation.

The fan coils are also set up for sec-

A fan coil supplies two ducts feeding this long slot grill on the upper floor that satisfies the supplemental cooling/humidity extraction for the main floor kitchen, dining and living area.

ondary heating, plus Grimshaw shares how during the shoulder seasons, on cool mornings when you don’t want to start putting heat into the slabs because

by two o’clock you’ll need cooling, there are settings where if you want a little bit of heat in the house, you don’t put it into the floors, you can provide heating with

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Modern Hydronics only the fan coils.

Overall, Grimshaw believes strongly in both the versatility and the comfort provided through hydronic heating and cooling. “Radiant floor heating is a very comfortable system, and I’ve lived with radiant heating systems in every house I’ve built. But I would venture to say that radiant for cooling is as comfortable, if not more, than a regular cooling system that’s blowing air all over the place.”

CONTROLS FIRST Grimshaw recalls the early days of his geothermal contracting business designing and installing systems with hydronics and fan coils and then configuring a bunch of relays to control the system.

He now appreciates taking a more controls forward approach, and serving as a test site for the new smart controls for this project made the process much cleaner to install from the start.

He notes that by preparing the controls setup ahead, along with a wiring diagram, allows the rough-in team to wire early (no fishing wires through finished walls) and the journeymen techs also know how many wires are running to each unit.

“The controls forward approach is just thinking about the project and piping it how it needs to be piped, wiring it and controlling it, and doing it all up front.

And it really creates a lot less stress,” says Grimshaw.

A COOL TREND While air-to-water heat pumps are still emerging on the Canadian residential market, both Grimshaw and Fallahkhair see promising growth for hydronic radiant heating and cooling.

“In North America, air conditioning has become an expected feature in residential construction rather than a luxury,”

says Fallahkhair. “Historically, hydronic systems have been viewed primarily as a heating system only, contributing to a decline in hydronic share of the residential HVAC market. Projects such as this demonstrate how modern controls, heat pumps, fan coils and radiant floor cooling can work together as an integrated comfort solution.”

Grimshaw agrees, adding that he’s seeing some growth in his market. “It’s the more forward-thinking contractors out there who are willing to look at new designs,” he says.

“The nice thing is that we’re showing those young contractors — the ones that used to be just the plumbing and heating guys who would pass on the cooling to the HVAC air-to-air guys — how to become cooling techs. We show them the controls and fan coils and let them know, ‘Hey, you don’t have to give that cooling work away anymore.’” <>

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HYDRONICS PRODUCTS

Modern Hydronics

NTI’s Verta Series of air-to-water heat pumps feature R32 refrigerant and deliver high water temperatures even in low ambient conditions. The outdoor Verta monobloc heat pump requires a wall-hung indoor unit with two options: the Hydrobox, with a 7-in. touchscreen and main control board that includes a 3kW backup heater, a circulation pump and a 3-way valve for DHW; or the Control Box which also has a 7-in. touchscreen and all necessary field wiring.

The Daikin ALTHERMA 3 H HT high-tempera-

ntiboilers.com

ture, air-to-water heat pump includes a monobloc outdoor unit plus an indoor unit with water lines running indoors. The outdoor unit uses

Hydronics Design Studio

R32 refrigerant, and the system operates down

Professional Version 3.0

By Mario Restive & John Siegenthaler

to -28C/-18F for heating. And the leaving water temperature can be up to 70C/158F. It uses

Tw = Tair + (Tin − Tair ) e

(

string segment 1

segment 2

inverter-driven compressors and fan motors,

⎡ ⎤ aL −⎢ ⎥ ⎣ 8.01Dc( f ) ⎦

H L = ⎡⎣α cL ⎤⎦ f 1.75

segment 3

)

q = ⎣⎡8.01Dc ⎤⎦ ( f )(∆T )

Copyright © 2025 By M. Restive and J. Siegenthaler Appropriate Designs, Holland Patent, NY

and with insulation surrounding the compressor and double sound reduction plates it can operate quietly, down to 41 dBA.

Version 3.0 of the Hydronics Design Studio, a

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Modern Hydronics

IMAGE COURTESY OF GRIDWORKS CONSULTING

IT’S ABOUT TIME

Figure 1. A graphing showing wholesale electricity pricing fluctuations for a utility in Maine.

As the industry continues down the road to electrification, there are ways to maximize hydronics system designs and minimize electrical costs.

BY JOHN SIEGENTHALER

M

any utilities in North America are increasing the amount of renewably generated electricity available to their grid. It mostly comes in the form of utility-scale solar photovoltaic systems and large wind turbines.

At times these intermittent sources, when combined with base generation, produce more electrical power than needed within the utility’s service territory. For utilities with large amounts of solar PV electric generation these “surges” in available power can occur under sunny mid-day conditions, or perhaps on weekends or holidays when normal electrical demand is lower.

For utilities that get power from large wind farms surplus output might occur on windy nights when demand is low but all the multi-megawatt turbines are producing at full capacity.

It might seem intuitive that when such “over generation” occurs the solution is MH16

FALL 2026

to shut down some of the generation equipment, but that’s not as simple and quick as turning off a switch. When a wind turbine is not producing power its economic return on investment is degrading, and its contribution to renewably generated electricity is zero.

A similar scenario applies to hydropower. The solution is not to curtail generation, it’s to find ways of off-loading surplus power to loads that can accept it.

One way to deal with over generation without curtailing renewably sourced electricity generation is to reduce the wholesale cost of that electricity and offer it for sale to other utilities.

Some utilities use data for load trends and weather forecasts to establish wholesale pricing for electricity 48 hours in advance. Other utilities can then decide what amount of electrical energy (megawatt•hours) they need, and what price they are willing to pay for that power.

MODERN HYDRONICS

This “transactive” approach to grid management has been used for years. It’s also proving to be a key concept for optimizing the use of renewably sourced electricity.

DEAL, OR NO DEAL To entice other utilities to purchase surplus power the utility wanting to sell it sets its “willing to sell” wholesale price for each of the next 48 hours. In some cases that “price” can be negative. That’s right; at the wholesale level there are times when one utility will pay another utility to accept their surplus power. This has occurred in utility districts with abundant solar and wind power generation, and in some cases the negative pricing can last for several hours.

Figure 1 shows an example of how the wholesale price of electricity (in $/ Megawatt•hour) can vary (this example is from central Maine). Any portions of the graph below 0 represent negative prices Continued on MH18 WWW.HPACMAG.COM

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TIME OF USE existing oil-fired boiler (retained as backup only) PRV indoor heat pump R-134a zone supplies

(P2) air separator

(P4)

(MV3)

A

B

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ECM circulator 0-10 VDC control

zone valves

(T1)

(PD)

1" copper

refrigerant line set

PRV

AB Outdoor heat pump R-410A

anti-condensation valve

(MV1)

1" copper

(P1) zone returns

119 gallon buffer tank

3-way valve allows for recirculation at startup and during defrosting

magnetic dirt separator

20 gallon thermal mass tank for defrosting

make-up water assembly

Tsr

62 gallon exp. tank

valves for testing 2-pipe vs. 3-pipe buffer configuration

ECM circulator 0-10 VDC control w/ internal check valve

discharge (MV2)

charge

air vent w/ isolation ball valve

(Psd)

TANK #3

discharge

charge

temperature sensors

TANK #2

TANK #1

electric elements sparge tubes

Figure 2. A complex piping concept used to take advantage of low electricity pricing.

(LMP = locational marginal pricing).

To sell access electricity there has to be a willing buyer and a way to move that power. The latter can be a major constraint for some utilities. The physical limits of the local power grid cannot always accommodate the need to wheel all surplus power.

TOO MUCH OF A GOOD THING Imagine what you could do with an air-towater heat pump capable of producing water at temperatures of 170F or more, thermal storage, and a retail utility rate that sets the price of electricity on an hourly basis 48 hours into the future.

During some of those hours, electricity might cost upwards of $0.40/ kWhr. At MH18

FALL 2026

119 gallon thermal storage tanks (in series)

other times it might be less than $0.07 / kWhr. And at still other times it might be less than $0.00 / kWhr! I suspect your first thought is to purchase the electricity when its price is lowest, convert it into heat using the heat pump, and then store that heat in insulated tanks of water. Well, you’re on the right track, but it gets a bit more complicated.

First, there must be assurance that the building being heated by this approach has consistent comfort, regardless of fluctuations in electricity prices.

Another consideration is available thermal storage capacity and the associated temperatures (i.e., you can’t store heat at 180F if the heat pump can only proMODERN HYDRONICS

duce 170F).

Another is the limitation of the heat emitters (i.e., you can’t drain storage down to 100F, if the heat emitters can’t sustain building comfort using those relatively low water temperatures).

Although the overall concept of such a system makes sense, the details of how to control it in an optimal way can be complex.

Research is currently underway in Northern Maine dealing with the above multi-variable scenarios, and modern hydronics technology is being used to reduce dependency on fuel oil by leveraging the advantages of this “transactive” management of electricity.

The research involves multiple instalWWW.HPACMAG.COM

PHOTO COURTESY OF GRIDWORKS CONSULTING

Modern Hydronics

Figure 4. Electric water heaters used as thermal storage tanks and piped in series.

Figure 3. The oil-fired boiler retained for back-up.

lations in existing “average” homes in central Maine. Those systems have extensive control and monitoring hardware. They were built to demonstrate what’s possible, and the results to date have been both impressive and encouraging.

The illustration in Figure 2 shows the piping concept for one of the installations.

These systems use high temperature “cascade” air-to-water heat pumps capable of producing 180F leaving water temperature. Those cascaded units have an outdoor heat pump running on R-410A refrigerant that routes refrigerant from its condenser to the evaporator of an indoor heat pump running on R-134A refrigerant.

The outdoor unit “lifts” the temperature of the refrigerant from below that of outdoor air to an intermediate level. The indoor heat pump “amplifies” that temperature higher than what a current generation air-to-water heat pump, running a single refrigerant cycle, can create. High enough to transfer heat to water at 180F. One of these cascade heat pumps was retrofitted to a system that had an oil-fired boiler serving 4-zones of fin-tube baseboard. The existing boiler, seen in Figure 3, was retained as a backup heat source.

The initial configuration of the system includes a 119-gallon buffer tank, along with three 119-gallon electric water heaters used as thermal storage tanks piped in series (see Figure 4).

This series piping configuration allows good temperature stratification. The coolest water is in the lower portion of the right tank, while the hottest water is in the upper portion of the left tank. All three tanks include “sparge tube” diffusers to minimize mixing currents that would otherwise disrupt temperature stratification.

The electric elements in these tanks are currently not used, but could be configured to capture low cost electrical energy beyond that used by the heat pump.

If used this way they can respond instantly, which is imporWWW.HPACMAG.COM

tant since some transactive electrical offerings might only last a few seconds.

Looking back at Figure 2, to put heat into storage the heat pump operates along with its circulator (P1).

Motorized valve (MV1) is closed, forcing flow into the upper portion of thermal storage tank #1. Water from the lower portion of tank #3 flows back to the heat pump through motorized valve (MV2).

During this time the buffer tank can provide some heat delivery to the distribution system. However, if the buffer tank runs low on heat valve MV1 opens to allow the buffer to be heated from the heat pump. This ensures that comfort is not compromised in the process of charging thermal storage.

To remove heat from thermal storage, circulator (P2) operates at variable speed, which is controlled by a 0-10 VDC signal from the system’s controller.

The flow of hot water passes through valve (MV1). The heated water from thermal storage can blend with water from the upper portion of the buffer tank as required to maintain a target supply water temperature to the heat emitters. That target temperature is based on outdoor reset.

TINY TANK One “nuance” of many air-to-water heat pumps is that they turn on their circulator 2-3 minutes prior to starting their compressor. During this time the heat pump’s internal controller measures flow rate to verify that adequate and stable flow is present prior to starting the compressor(s).

Although this action is prudent in protecting the heat pump against inadequate flow, it can destroy beneficial temperature stratification in the thermal storage subsystem by circulating water from the lower portion of the coolest tank to the upper portion of the hottest tank.

One work around is to “recirculate” flow exiting the heat

MODERN HYDRONICS

Continued on MH22 FALL 2026

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TIME OF USE

THE BRAINS Over the last several decades the controls used in hydronic heating systems have ranged from crude bimetal switches, to capillary tube aquastats, to little glass tubes containing a few drops of mercury, and finally to microprocessors.

The systems being tested in Maine take it a step further. They use a Raspberry Pi microcomputer as both a local controller and a means of accessing online energy cost and weather forecasting data.

That little computer costs about $40.

It accepts inputs from several temperature sensors and flow meters and uses that information along with some very specific programing to coordinate the operation of every valve, circulator or other controllable device in the system. It’s also used to log and report system performance information.

The Raspberry Pi can’t directly power MH22

FALL 2026

Raspberry Pi™ microcomputer relay board for 24VAC and line voltage outputs Figure 5. A Raspberry Pi microcomputer and a relay board it drives.

24VAC or line voltage devices, but that’s easily handled by using the digital outputs from the Raspberry Pi to drive relays that switch 24VAC and line voltage devices on and off.

Figure 5 shows the Raspberry Pi microcomputer and one of the relay boards it drives.

You may be thinking, what happens to this system if the Internet connection is lost?

The people conducting the research have planned for that contingency. If the Internet connection is lost the system automatically goes into a safe “local control” mode. And if the Raspberry Pi computer fails the system automatically reverts to its original oil-fired boiler mode.

WHY THIS IS RELEVANT Research systems are built to test concepts and find potential “weak points” in those concepts. The process comes down to test, observe, refine and repeat.

The systems described above are works in progress. Still, from what I’ve seen so far their operation is impressive.

One system was able to maintain stable comfort in a home through nights with -5F/-20C outside temperatures while running the heat pump only during offpeak (low cost) electrical periods.

The data indicates minimal variation in the hot water temperature supplied to the zone circuits while the heat pump MODERN HYDRONICS

was operating as well as during on-peak electrical periods up to 5 hours long, when the heat pump was off.

I’ve had the pleasure or working with the team that designed, installed and operated these research systems, and in my opinion they are exploring concepts that are far beyond what the current North American hydronics industry deals with on a day to day basis.

Continued work on these systems will lead to refinements in both the hardware and software used for transactive energy management. Test results from these systems may eventually lead regulators to approve different electric rate structures that could benefit both the customer and the utility.

Those results could also be laying the groundwork for how hydronics technology is applied in the future. These systems also demonstrate that modern hydronics technology is an ideal “media” for leveraging more complex electrical rate structures that benefit both the customer and the utility. <> John Siegenthaler, P.E., has over 40 years of experience designing modern hydronic heating systems and is the author of Modern Hydronic Heating (4th edition) and Heating with Renewable Energy (visit hydronicpros.com).

WWW.HPACMAG.COM

IMAGE COURTESY OF GRIDWORKS CONSULTING

pump back into the heat pump until the leaving water temperature is slightly higher than the temperature at the top of the “hot” thermal storage tank. From a piping standpoint this is accomplished using a 3-way motorized valve.

Another situation that can disrupt temperature stratification in storage is when the heat pump goes into defrost.

During this time the heat pump operates in cooling mode, pulling heat from “something” inside the building, and using it to melt frost on the outdoor coil.

For the system in Figure 2 that “something” is a small tank piped so that it’s part of the recirculation circuit through motorized valve (MV3).

A 20-gallon tank can supply defrost heat for a nominal 4-ton heat pump for about eight minutes, which is typically adequate to complete a defrost cycle.

Other defrosting control scenarios are possible, including temporarily operating all the zones to scavenge heat from the building, or tapping into the thermal mass in the lower portion of the coolest thermal storage tank.

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